Growing nitrous oxide levels explained

Published this week in the journal Nature Geoscience, the record is drawn from atmospheric sampling at the Cape Grim Baseline Air Pollution Station, Tasmania, and air extracted from the Antarctic ice sheet

Its significance, says co-author CSIRO’s Dr David Etheridge, will be its contribution to the development of emissions protocols as countries step up their monitoring of gases contributing to global warming and ozone depletion.

Nitrous oxide, N₂O, is produced naturally by microbial activity in soils and the oceans as well as by agriculture. With a lifetime in the atmosphere of around 120 years, it is eventually broken down by oxidation in the stratosphere.

Scientists have measured a 20% increase in atmospheric nitrous oxide since 1750.

Dr Etheridge said that through analysis of isotopes, which are different forms of the nitrogen and oxygen comprising nitrous oxide, researchers identified that the increase of nitrous oxide was largely from agricultural sources.

“Until now the balance between the stratospheric sink and the oceanic source of nitrous oxide has been a key uncertainty in understanding nitrous oxide variations at the Earth’s surface.”

CSIRO’s Dr Etheridge

“Through these measurements and a close analysis of the record we can also see seasonal cycles in N₂O’s isotopic composition, as well as large year-to-year variations and long-term trends,” he said

He said the unique record of seasonal cycles allows scientists to trace the production and loss of nitrous oxide, from its origins in the ocean and soils to its movement through the stratosphere where it contributes to breaking down ozone.

“Until now the balance between the stratospheric sink and the oceanic source of nitrous oxide has been a key uncertainty in understanding nitrous oxide variations at the Earth’s surface.

“This new evidence of the budget nitrous oxide allows us to better predict its future changes– and therefore its impacts on climate and stratospheric ozone depletion – for different scenarios of fertiliser use in support of rising populations and increased production for bio-energy,” he said.

Dr Etheridge said the record demonstrates the value of Australia’s investment, through CSIRO and the Bureau of Meteorology, in a world-class atmospheric monitoring station since the 1970s, with its underlying vision of establishing an air archive which can be measured as new technologies are developed.

Adding to this is the Australian Antarctic Division’s glaciology program that accesses a natural archive of the past atmosphere, at its primary drilling site Law Dome.

Dr Etheridge said that better estimates of the magnitudes and geographic distribution of N2O sources and how they change over time help formulate sound emissions policies.

Led by University of California-Berkeley’s Dr Sunyoung Park, Australian co-authors included CSIRO scientists at the Centre for Australian Weather and Climate Research, Drs Paul Fraser, Paul Krummel, Ray Langenfelds, Paul Steele, Cathy Trudinger, and Dr Tas van Ommen, from the Australian Antarctic Division.

The Cape Grim Air Pollution Station’s scientific research is jointly managed by the Bureau of Meteorology and CSIRO Marine and Atmospheric Research. The Australian Antarctic Division and the Australian Antarctic Science Program supported access and drilling at Law Dome.

The research was supported through the Cape Grim Science Program, the Australian Climate Change Science Program and the Department of Climate Change and Energy Efficiency.